This invention relates generally to integrated circuit processes and fabrication, and more particularly, to a precursor and method, having enhanced temperature stability, used to deposited copper on selected integrated circuit surfaces.
The demand for progressively smaller, less expensive, and more powerful electronic products, in turn, fuels the need for smaller geometry integrated circuits (ICs) on larger substrates. It also creates a demand for a denser packaging of circuits onto IC substrates. The desire for smaller geometry IC circuits requires that the interconnections between components and dielectric layers be as small as possible. Therefore, research continues into reducing the width of via interconnects and connecting lines. The conductivity of the interconnects is reduced as the area of the interconnecting surfaces is reduced, and the resulting increase in interconnect resistivity has become an obstacle in IC design. Conductors having high resistivity create conduction paths with high impedance and large propagation delays. These problems result in unreliable signal timing, unreliable voltage levels, and lengthy signal delays between components in the IC. Propagation discontinuities also result from intersecting conduction surfaces that are poorly connected, or from the joining of conductors having highly different impedance characteristics.
There is a need for interconnects and vias to have both low resistivity, and the ability to withstand process environments of volatile ingredients. Aluminum and tungsten metals are often used in the production of integrated circuits for making interconnections or vias between electrically active areas. These metals are popular because they are easy to use in a production environment, unlike copper which requires special handling.
Copper (Cu) would appear to be a natural choice to replace aluminum in the effort to reduce the size of lines and vias in an electrical circuit. The conductivity of copper is approximately twice that of aluminum and over three times that of tungsten. As a result, the same current can be carried through a copper line having nearly half the width of an aluminum line.
The electromigration characteristics of copper are also much superior to those of aluminum. Aluminum is approximately ten times more susceptible than copper to degradation and breakage due to electromigration. As a result, a copper line, even one having a much smaller cross-section than an aluminum line, is better able to maintain electrical integrity.
There have been problems associated with the use of copper, however, in IC processing. Copper pollutes many of the materials used in IC processes and, therefore barriers are typically erected to prevent copper from migrating. Elements of copper migrating into these semiconductor regions can dramatically alter the conduction characteristics of associated transistors. Another problem with the use of copper is the relatively high temperature needed to deposit it on, or removing it from, an IC surface. These high temperatures can damage associated IC structures and photoresist masks.
It is also a problem to deposit copper onto a substrate, or in a via hole, using the conventional processes for the deposition of aluminum when the geometries of the selected IC features are small. That is, new deposition processes have been developed for use with copper, instead of aluminum, in the lines and interconnects of an IC interlevel dielectric. It is impractical to sputter metal, either aluminum or copper, to fill small diameter vias, since the gap filling capability is poor. To deposit copper, first, a physical vapor deposition (PVD), and then, a chemical vapor deposition (CVD) technique, have been developed by the industry.
With the PVD technique, an IC surface is exposed to a copper vapor, and copper is caused to condense on the surfaces. The technique is not selective with regard to surfaces. When copper is to be deposited on a metallic surface, adjoining non-conductive surfaces must either be masked or etched clean in a subsequent process step. As mentioned earlier, photoresist masks and some other adjoining IC structures are potentially damaged at the high temperatures at which copper is processed. The CVD technique is an improvement over PVD because it is more selective as to which surfaces copper is deposited on. The CVD technique is selective because it is designed to rely on a chemical reaction between the metallic surface and the copper vapor to cause the deposition of copper on the metallic surface.
In a typical CVD process, copper is combined with a ligand, or organic compound, to help insure that the copper compound becomes volatile, and eventually decomposes, at consistent temperatures. That is, copper becomes an element in a compound that is vaporized into a gas, and later deposited as a solid when the gas decomposes. Selected surfaces of an integrated circuit, such as diffusion barrier material, are exposed to the copper gas, or precursor, in an elevated temperature environment. When the copper gas compound decomposes, copper is left behind on the selected surface. Several copper gas compounds are available for use with the CVD process. It is generally accepted that the configuration of the copper gas compound, at least partially, affects the ability of the copper to be deposited on to the selected surface.
Cu.sup.+2 (hfac).sub.2, or copper (II) hexafluoroacetylacetonate, precursors have previously been used to apply CVD copper to IC substrates and surfaces. However, these Cu.sup.+2 precursors are notable for leaving contaminates in the deposited copper, and for the relatively high temperatures that must be used to decompose the precursor into copper. Currently, more success has been found with the use of Cu.sup.+1 (hfac) compounds to apply copper. Norman, et al., U.S. Pat. No. 5,322,712, discloses a (hfac)Cu(tmvs), or copper hexafluoroacetylacetonate trimethylvinylsilane, precursor that is the industry standard at the time of this writing. Alternately, tmvs is known as vtms, or vinyltrimethylsilane. This precursor is useful because it can be used at relatively low temperatures, approximately 200.degree. C. In addition, the film resisitivity of copper applied with this method is very good, approaching the physical limit of 1.7.mu..OMEGA.-cm. However, the adhesiveness between copper deposited with this precursor and the surface to which it is deposited is not always good. Also, the precursor is not especially stable, and can have a relatively short shelf life if not refrigerated. Various ingredients have been added to (hfac)Cu(tmvs) to improve its adhesiveness, temperature stability, and the rate at which it can be deposited on an IC surface. A co-pending application Ser. No. 08/745,652 filed Nov. 8, 1996, entitled "Cu(hfac)TMVS Precursor With Water Additive To Increase The Conductivity Of Cu And Method For Same", invented by Nguyen et al., Attorney Docket No. SMT 244, a assigned to the same assignee as the instant patent, discloses and precursor and method of improving the electrical conductivity of Cu deposited with (hfac)Cu(tmvs).
It is generally acknowledged in the industry that (hfac)Cu(tmvs) becomes unstable, and begins to decompose, above 35.degree. C. Use of a (hfac)Cu(tmvs) precursor stored at this temperature leads to undesirable process results. Typically, the precursor is a liquid at room temperature, and must be converted to a vapor form. In interacting with a heated target surface, the vaporized precursor first cleaves the tmvs ligand, and then the hfac, leaving Cu on the target surface. During this process a disproportionation reaction occurs in which uncharged atoms of Cu are left on the surface, while volatile forms of Cu.sup.+2 (hfac).sub.2 and the tmvs ligand are exhausted through the system.
As an unstable precursor is heated to a vapor, the tmvs ligand cleaves unevenly from the precursor, some cleavage, or decomposition, occurs at low temperature, and some at higher temperatures. Because the precursor decomposes at low temperatures, the precursor vacuum pressure, or partial pressure, remains undesirably low, resulting in low Cu deposition rates, uneven surfaces, and variances in surface conductances. The effectivity of (hfac)Cu(tmvs) stored at temperatures lower than 35.degree. C. is also unpredictable. "Fresh" batch of precursor, or precursor stored at temperatures well below room temperature are used to guarantee predictable processes.
Various additives have been mixed with the (hfac)Cu(tmvs) precursor to improve its temperature stability. It is well known to blend hexafluoroacetylacetone (H-hfac), tmvs, and other chemical agents to improve temperature stability. Baum et al., in "Ligand-stabilized copper(I) hexafluoroacetylacetonate complexes: NMR spectroscope and the nature of the copper-alkene bond", J. Organomet. Chem., 425, 1992, pp. 189-200, disclose alkene groups affecting improvement in the stability of Cu precursors. They also speculate on the nature of sigma and pi bonds in the Cu-alkene bond of a (hfac)Cu(alkene) complex.
Choi et al., in "Chemical vapor deposition of copper with a new metalorganic source", Appl. Phys. Lett. 68 (7), Feb. 12, 1996, pp. 1017-1019, disclose trimethoxyvinylsilane (tmovs) as a ligand to improve the temperature stability of Cu(hfac). Using the tmovs ligand, precursor stability up to the temperature of 70.degree. C. is reported. However, the addition of oxygen atoms to the methyl groups of the ligand is still experimental. That is, the method has not been refined for production environments. There is also concern that a precursor having a heavier molecular weight due to the addition of oxygen atoms to the ligand may yield unexpected premature decomposition problems as higher vaporization temperatures and lower system pressures are required. Further enhancement in the temperature stability of Cu(hfac) precursors is desirable, and the use of other ligands to improve stability remains an area of ongoing research.
It would be advantageous if a method were found of making a Cu(hfac) precursor stable over a wider range of temperatures, and to provide the precursor with a longer shelf life.
It would be advantageous if the ligand attached to the Cu(hfac) precursor would cleave at a consistent temperature. Additionally, it would be advantageous if the alkene ligand and the hfac would cleave at approximately the same temperature to yield consistent precursor decomposition.
It would be advantageous if the temperature at which the (hfac)Cu(ligand) compound decomposes could be increased, thereby increasing the precursor partial pressure, to deposit a thicker layer of copper on selected IC surfaces.
It would also be advantageous if a water additive could be blended with a thermally stable Cu precursor to improve the conductivity, and deposition rate, of the deposited copper.
Accordingly, a volatile Cu precursor compound for the chemical vapor deposition (CVD) of Cu to selected surfaces is provided. The precursor compound comprises Cu.sup.+1 (hexafluoroacetylacetonate), and a (methoxy)(methyl)silylolefin ligand. That is, the ligand comprises a combination of methyl and methoxy groups, bonded to silicon, which strike a balance between a precursor having the electron donation capability of methoxy in the ligand, and a precursor having a lesser molecular weight due to the methyl in the ligand. The electron donation capability of the oxygen in the (methoxy)(methyl)silylolefin ligand provides a secure bond between the Cu and the (methoxy)(methyl)silylolefin ligand as the compound is heated to vaporization temperature.
In one alternative of the preferred embodiment, the (methoxy)(methyl)silylolefin is dimethoxymethylvinylsilane (dmomvs), whereby the two oxygen atoms of the dimethoxymethyl group donate electrons to Cu to increase the temperature stability of the precursor. In another alternative of the preferred embodiment, the (methoxy)(methyl)silylolefin is methoxydimethylvinylsilane (modmvs), whereby the oxygen atom of the methoxydimethyl group minimally suppresses the precursor volatility. Therefore, a ligand providing electrons from either one, or two, oxygen atoms is disclosed.
The preferred embodiment further includes an addition to the compound to create a precursor blend. The precursor blend further comprises a water vapor having a vacuum partial pressure. The water vapor is blended with the precursor so that the partial pressure of the water vapor is generally in the range of 0.5 to 5% of the precursor partial pressure, whereby the addition of water vapor to the precursor increases the rate of Cu deposition and the electrical conductivity of the deposited Cu.
A method for applying CVD Cu on a selected surface is also provided. The method comprising the steps of: a) exposing each selected Cu-receiving surface to a volatile Cu precursor compound including Cu.sup.+1 (hexafluoroacetylacetonate) and a (methoxy)(methyl)silylolefin ligand at a predetermined vacuum pressure; and, b) while continuing to carry out step a), depositing Cu on each Cu-receiving surface. The bond between the (methoxy)(methyl)silylolefin ligand and Cu prevents the decomposition of the precursor at low temperatures.